The present invention relates generally to electrochemical energy storage devices and more specifically to alkaline electrochemical capacitors incorporating high surface area electrodes.
The desirability and utility of providing efficient electrical energy storage devices is well known. As technological advances continue, more and more products and devices rely on an efficient, economical supply of electrical energy for operation. Moreover, as those products are refined and often miniaturized, the desirability for an efficient, compact, high powered energy storage device becomes even greater.
Generally, in order to maintain or increase the electrical energy output of a storage device while simultaneously decreasing its size, the energy density (energy per unit volume) of the device must be increased. The electrochemical energy in a storage device such as an electrochemical capacitor depends on the total electrode surface area which is in contact with the electrolyte. Accordingly, one way to increase the energy available without increasing the physical size of the device is to increase the surface area of the electrodes. Further, the ionic conductivity or activity of the electrolyte can be increased to enhance power output. But, as known to those skilled in the art, the higher activity electrolyte solutions can lead to decay of the electrode material.
In addition to the desirability of providing a storage device having small physical size and high-energy output, is the desirability of providing a sealed, environmentally stable energy storage device. An advantage to such a device would lie in its ability to be manufactured as a sealed package, facilitating its incorporation into larger systems and high reliability products.
Varied configurations of high-density energy storage devices have been developed to date. As an example, U.S. Pat. No. 5,626,729 to Thompson et al. entitled xe2x80x9cModified Polymer Electrodes for Energy Storage Devices and Method of Making Samexe2x80x9d discloses an electrochemical capacitor utilizing non-noble metal substrates upon which a nitride layer is formed. An electrochemically stable pseudocapacitive polymer coating is deposited on the nitride layer and an electrolyte disposed in-between. A limitation inherent in this device is that the chemically doped polymers incorporated therein tend to suffer progressive decay when used in electrolyte solutions strong enough to provide the high power density desired from capacitors.
U.S. Pat. No. 5,680,292 to Thompson, Jr. et al. entitled xe2x80x9cHigh Surface Area Nitride, Carbide and Boride Electrodes and Methods of Fabrication Thereofxe2x80x9d discloses high surface area electrodes wherein a conductive ceramic coating is formed on a molybdenum, tungsten or vanadium metal substrate. A separator is impregnated with an acidic ionically conductive electrolytic solution such as aqueous sulfuric acid. The electrodes pertinent to such processing methods are electrochemically unstable in strong alkaline electrolyte. These electrodes would act as passive electrical insulators in an alkaline electrolyte upon application of a small (half-volt) positive charge, thus precluding their use in alkaline capacitors.
An example of a commercially available device is the capattery manufactured by the Evans Capacitor Company, East Providence, R.I. The capattery is a capacitor incorporating carbon powder electrodes in conjunction with a sulfuric acid electrolyte. Although this type of capacitor has been useful in certain applications, it suffers from the limitation that carbon dioxide gas is released as a byproduct of the charge/discharge cycling and must be vented. As a result, the capacitor package can""t be hermetically sealed. Moreover, since the upper operating temperature limit is 85xc2x0 C. and since the device must be vented, it would fail by drying out when exposed to temperatures exceeding its operational limit.
A need exists therefore for an improved high-density electrical energy storage device. Such a device would be compact, capable of being hermetically sealed and fabricated from readily available materials and processes.
It is therefore a primary object of the present invention to provide an electrical energy storage device overcoming the limitations and disadvantages of the prior art.
It is still another object of the present invention to provide an electrochemical capacitor that takes advantage of the desirable qualities of strong alkaline electrolyte solutions to provide high energy density deliverable at high power density.
It is yet another object of the present invention to provide an electrochemical capacitor that is sealed and capable of long-term cyclic operation over a wide range of operating temperatures.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a high-density alkaline electrochemical capacitor and method of fabrication is described. The method of the present invention includes fabricating a pair of titanium powder electrodes. In the preferred embodiment, the powder electrode is comprised of titanium nitride powder obtained by reacting titanium hydride powder with a controlled flow of ammonia gas for one hour at a temperature of 700xc2x0 C. Alternatively, the powder electrode can be obtained by reacting titanium nitride powder with ionized ammonia. This reaction can be completed at room temperature. Advantageously, the titanium nitride electrodes, by virtue of their powder composition, have a high surface area, greatly enhancing the overall operation of the capacitor cell.
A porous separating membrane for containing an electrolyte is affixed between the pair of titanium nitride powder electrodes. In the preferred embodiment, the electrolyte solution is 7.6M (14.7 pH) potassium hydroxide. Advantageously, this strong alkaline electrolyte helps provide the desirable high-density energy storage quality of the capacitor cell fabricated according to the method of the present invention.
The capacitor cell is capped or sealed by a pair of conductive titanium nitride terminations. In the preferred embodiment, these terminations are fabricated by heating a thermoplastic elastomer to a temperature sufficient to melt and adding finely ground titanium nitride or titanium carbonitride powder thereto. Alternatively, these terminations may be fabricated by dissolution of a solvent processible elastomer, which is allowed to cure following addition of fine particulate titanium nitride or carbonitride. Such conductive elastomeric terminations perform the dual function of sealing the capacitor as well as being electrically conductive so as to facilitate the ready transference of electrical energy into and out of the capacitor cell. The cell conductivity may be further enhanced by RF magnetron sputter coating of the conductive elastomer with titanium nitride or carbonitride.
Advantageously, this capacitor cell construction, by making use of the alkaline electrolyte, facilitates cyclic operation without any release of gaseous byproducts. This permits hermetic sealing of the capacitor thereby contributing to highly reliable device operation and longevity by preventing dry out of the electrolyte solution. It renders this capacitor capable of long-term cyclical operation over a wide range of operating temperatures (xe2x88x9255xc2x0 C. to +100xc2x0 C.) while providing high-density energy storage. Moreover, by virtue of its being hermetically sealed, and the attendant long-term cyclic operation, this capacitor can be permanently incorporated into a vast array of devices without concern for routine replacement. As can be appreciated, this contributes to enhanced system reliability as well as reduced operational costs.